Electrical load management systems for allowing an electrical utility to control the load on an electrical distribution system are known in the art. These systems divert energy requirements to minimize electrical black-outs or "brown-outs". For example, U.S. Pat. No. 4,190,800 to Kelly, Jr. et al., entitled "Electrical Load Management System", assigned to the same assignee as the present invention, describes an electrical load management system wherein a utility command center monitors the use of electrical power and, when peak demand periods occur, transmits coded information by radio from the command center to remote receivers mounted proximate the electrical loads. In this patent, the transmitted signal includes address and command information that is decoded at the receivers. Receivers addressed by the command center pass command information over the distribution lines to the electrical loads, and thereby control the operation of the customers' power consuming devices.
Other load management systems utilize separate radio receivers at each customer's location, rather than providing a receiver at the distribution transformer as in the aforementioned patent. Examples of this type system include the types DCU-1120, -1170, -1180, -1190, and -S2000A utility radio switches, otherwise described as digital control units or load control switches, manufactured by Scientific Atlanta, Inc., Atlanta, Ga., and the type REMS-100 radio switch manufactured by General Electric, King of Prussia, Pa. These utility radio switches incorporate an FM receiver that can receive a transmitted signal up to about 25 miles from a transmitter site located at a command center. The transmitter issues commands to temporarily remove power from a selected load. This self-contained receiver is typically mounted on or immediately adjacent to the electrical loads under control, and receives its power from the line that feeds the controlled loads. Switches, jumpers, or other means contained within the receiver configure the receiver to respond only to a particular address or set of addresses, so that different geographical areas, types of appliances, or numbers of consumers may be separately controlled.
Such a load management system reduces peak power demand and therefore the utility need not generate or import as much additional power. In order to induce more customers to participate in their load management programs, utilities typically offer price incentives to participants.
However, if the load management device has been bypassed through tampering or by a service technician working on a controlled appliance, the utility cannot promptly discover the problem. Air conditioning technicians in particular may not understand the function of the load management device. They often disconnect the air conditioner's thermostatic control circuit from the contacts of the device as a troubleshooting procedure. After the trouble with the air conditioner has been solved, the technician may not reconnect it to the load management device, leaving the utility with no control over the appliance. The utility will continue to discount the participant's bill without the benefit of disconnecting the appliance during peak power consumption.
As a result of this problem, utilities have sought a tamper detection system for several years. One early attempt provided a circuit for monitoring the coil of the load switch relay for continuity. While detecting failure of the relay, that method would not determine whether the contact circuit was still connected to the load. Another method was to measure the voltage induced in the load switch relay coil across the contacts when the load was reconnected. It was common for the load, such as a hot water heater, to be drawing current upon reconnect when the appliance had been cut off for a period of time by the load management device. Therefore, voltage would be induced in the coil by the load current passing through the contacts of the load management device. A microprocessor in the system would determine whether the ratio of reconnects with current to those without current was acceptable. An unacceptable ratio would be taken to evidence of tampering. Because this method depended on load management activity to have a monitored event, it was effective only when the utility used the load management system often. Utilities therefore have had a need for a tamper detection system that operates without any activity of the load management device.
Thus, there has been a need for a low cost tamper detection system that is relatively simple and cost effective to implement, operates without any activity of the load management device, warns the utility promptly when a load has been disconnected from the load management device, and detects disconnection even when the management device itself is powered up and working properly.